1. Field of the Invention
The present invention relates to electrical circuits for protecting an electric motor from damage caused by stalling the motor.
2. Background Art
Small, low current electric motors are used in a myriad of applications in which a reversible drive is required. Examples of some applications include vehicle seat positioning systems, seat recliners, window regulator drives, mirror positioning systems, and the like. For example, a reversible motor may be used in a vehicle seat to control the fore-and-aft positioning of the vehicle seat in a track. The motor is used to adjust the seat forwardly or rearwardly to accommodate a seat occupant. Seats move within tracks between a maximum forward position and a maximum rearward position that is determined by stops defining the length of the track. When the motor drives the seat to either maximum position, the motor may stall if the switch controlling the motor is continued to be actuated after the seat reaches the maximum travel position. Stalling the electric motor may cause damage to the motor.
Prior art solutions to the problem of damage caused to motors by stalling include providing positive temperature coefficient (PTC) devices or circuit breakers in the motor drive control circuit to protect the electrical motor. PTC devices may tend to degrade with time. Circuit breakers are thermal devices that are subject to hysteresis effects that may cause intermittent seat operation. For example, if the switch is held until the motor stalls after the seat has been moved to maximum travel position, the circuit breaker through internal heating interrupts the supply of current to the motor. The circuit breaker cannot be reset until its temperature drops and resets the circuit breaker which may be perceived as a malfunction by the seat occupant.
Applicants' invention is directed to providing a simple and effective integrated motor control that protects the motor from damage in the event the motor stalls. Applicants' invention provides a system that has an adjustable threshold current level and also provides a robust system that may be repeatedly activated without degradation or adverse hysteresis effects. Applicants' invention is summarized below.